Unattended repeater system



Oct. 29, 1935'. E. l. GREEN i-:r Al. 2,018,851

UNATTENDED REPEATER SYSTEM y Filed June 9, 1934 z'sheets-sneet 1 Oct. 29, 1935. E. l. GREEN x-:r A1.

UNATTENDED REPEATER SYSTEM 2 Sheets-Sheet 2 Filed June 9, 1934 INVENTORS ATTORNEY Patented Oct. 29, 1935 UNITED STATES PATENT OFFICE UNATTENDED REPEATER SYSTEM Application June 9, 1934, Serial No. 729,923

16 Claims.

'I'his invention relates to transmission systems and more particularly to systems involving unattended repeater stations.

In the development of new types of transmission systems, particularly those employing wide ranges of frequencies, it is desirable to employ repeaters spaced at more frequent intervals. Because of the large number of repeaters involved, it is desirable that some of the repeaters be left unattended especially as repeaters have been developed which are sufliciently reliable in perfomance to be left unattended in suitable housings for considerable periods of time.

,Regardless of the degree of reliability which is attained in the development of such unattended repeaters, however, the possibility of an occasional failure can never be entirely ruled out. Where more than one unattended repeater is employed between adjacent attended stations, it is desirable, in the event of failure of one of the unattended repeaters, to have available some means whereby the attendant at an attended repeater station may readily determine which unattended repeater is inoperative. It is one of the purposes of the present invention to provide facilities whereby this information may be obtained at attended repeater points.

'Ihe indication at the attended repeater point may be based upon the ability of the unattended repeater to amplify (a) the voice, television, or other signals transmitted; or (b) the indication may be based upon the ability of the amplifier to transmit a pilot frequency or frequencies.

In a system having so few communication channels superimposed on it that the signals passing through the unattended repeater will be intermittent in character, it would be preferable to base the indication upon a pilot frequency or frequencies which are continuously transmitted.

Various types of signaling systems for indicating failure of a repeater and for determining the location of the repeater which has failed, may be provided. For example, the system may be arranged to transmit continuously through the unattended repeaters an alternating current which may be either the normal signaling currents passing over the system or a special pilot frequency. Each unattended repeater may be so related to a direct current path common to all of the unattended repeaters of the group, that upon failure of the repeater to operate, a change will be produced in the condition of the direct current path thereby producing a signal at an attended repeater station. The attendant upon observing the signal may then test over the direct current path to determine the point at which the change in condition has occurred, as, for example, by making resistance measurements.

'Ihe invention may also be embodied in an arrangement in which, when the repeater fails, the location of the repeater may be indicated at an attended station directly, without the necessity of any testing operation. For example, 10

instead of using a direct current path a plurality of pilot frequencies, one corresponding to each unattended repeater, may be transmitted over the circuit from the attended station through the various unattended stations. tended station is arranged to exercise control over the pilot frequency assigned to that particular station, and upon the failure of a particular unattended repeater that repeater will be identified by the fact thatthe pilot frequency corresponding thereto produces no indication at the indicating point.

Also, the invention may be embodied in an arrangement in which, instead of transmitting separate pilot frequencies or other signaling currents for testing purposes over each one-way transmission circuit of a two-way system, as is contemplated in the arrangements above described, a combined indicating arrangement for both directions may be used. For example, failure of the unattended repeater for either direction at a given repeater station may be caused to produce at an attended repeater an indication of the station at which the failure has taken place. An additional signal individual to each directional path and common to all of the unattended repeaters in tandem in that path, may be used to indicate failure of any repeater in the path. By means of the two indications the individual repeater which has failed may be ascertained.

While only arrangements corresponding to the second type of system above mentioned are disclosed herein, the general principles of the invention may be embodied in organizations of types other than those illustrated, without departing from the spirit of the invention as defined in the claims appended hereto. Systems of the rst type above described, are disclosed and claimed in the joint application of E. I. Green and F. A. Leibe, Serial No. 729,922 filed June 9, 1934, which bears a divisional relation to the present application. Systems of the third type above described, are disclosed and claimed in an Each unat- 1 application of F. A. Leibe, Serial No. 729,921, filed June 9, 1934:.

The invention will now be more fully understood from the following detailed description thereof when read in connection with the accompanying drawings in which Figure 1 illustrates an embodiment of the invention employing an auxiliary path for transmitting alternating current signals to the attended station for indicating the location of an unattended repeater which has failed; Fig. 2 illustrates a similar system in which the alternating path for indicating the location of the repeater which has failed is superimposed upon the main signal transmission circuit; Fig. 3 illustrates a modified arrangement in which the repeater which has failed at an unattended station exercises a different kind of control over the alternating current path, and in which the failure of the repeater is also used to cut in an alternate repeater automatically until an attendant has had time to go tothe unattended station to rectify the trouble; and Fig. 4 illustrates an arrangement in which pilot frequencies individual to each unattended repeater station are provided for locating the repeater which has failed by successively applying said frequencies to the circuit after the failure has been indicated by an alarm located at the attended station and common to all of the unattended stations.

Referring to Fig. l, L designates a portion 'of a transmission circuit having attended repeater stations X and Y with unattended repeater stations at points i, 2 and 3 between stations X and Y. An attended repeater AX is located at station X, and associated with the output of said repeater through a suitable transformer-is a rectier RX. As long as transmission passes through the repeater AX the rectifier Rx rectiiies a portion of the received transmission and therebyenergizes a relay. The received currents to be rectiiied may be either normal signaling currents, or, as previously stated, where the number of channels normally transmitted is too small to assure that some signal will be transmitted at all times, a pilot frequency may be transmitted over the line L to hold up the relay. If the repeaterv Ax or any portion of the transmission circuit ahead of said repeater should fail, there will be no current in the output of the rectifier Rx and the relay will be released to actuate the alarm signal Sx.

At station Y a similar rectiiier Ry is associated through a transformer with the output of the attended repeater Ay, and controls through its relay an alarm signal Sy. The operation of the alarm Sy will indicate failure of the repeater Ay, or failure in the circuit ahead of the repeater Ay.

At stations l, 2 and 3, unattended repeaters U1, U2 and Us, are provided. A rectifier R1 is associated by means of a transformer, with the output of the unattended repeater U1 and rectiiies a portion of the output thereof to hold up a relay. Similarly, a rectier R2 is associated with the output of the repeater U2 and a rectifier R3 is associated with the output of the repeater U3. Power for supplying the filament and other operating currents to the repeaters and rectifiers, may be supplied either from` a local source or from the line circuit L itself, where the line circuit is arranged to transmit the power. The power thus supplied at station I passes through apparatus P1 for regulating the control of the power to be supplied, thence to the repeater U1 and the rectifier R1 over the two paths indicated in the drawings. Apparatus P1 may be of any well known type of power supply equipment. Similar regulating equipments P2 and Ps are provided at stations 2 and 3.

In order to indicate failure of an unattended station as well as to indicate the location of the repeater which has failed, an auxiliary path L is provided. Associated with one end of this path at the station X three generators XG1, XGz and XGz are provided, and these generators may be connected through switches to the line L in order to apply thereto frequencies f1, f2, and f3 corresponding to the three unattended stations I, 2 and'3. At attended station Y three filters YF1, YF2 and YFa selective of frequencies f1, f2, and fa. respectively, are connected to the line L. The individual frequencies thus selected are applied to rectiiiers YR1, YRi` and YRE. Alarms YS1, YS2 and YSs are associated with the rectifiers and may be operated by the direct current in the output circuitv of the rectifiers in the same manner as the alarms Sx and Sy are operated. However, in order to simplify the drawings the relays in the output circuits of the rectiiiers and the contacts controlled thereby, for the alarm circuits, are not illustrated in detail.

In order to control the frequencies transmitted over the line'L to indicate repeater failure, tuned circuits are arranged to ce bridged across the line L at each of the stations i, 2, and For example, at station I tuned circuit TC1, tuned to the frequency f1, is controlled by the relay of the rectifier R1 so that when the relay fails the tuned circuit TG1 is bridged across the line L', thus providing a short circuit for the frequency f1 so that this frequency will be substantially suppressed at the station Y and the rectifier YR1 will actuate the alarm YS1. Similarly, tuned circuit TG2, tuned to the frequency f2, may be bridged across the line L by the relay associated with the rectifier R2 at station and at station 3 the relay associated with the rectifier R3 is arranged to connect tuned circuit TG3, tuned to the frequency f3, across the line L.

Upon failure of one of the attended repeaters, as, for example, repeater U2, the rectifier Ry will no longer receive current and, consequently, the alarm Sy will be actuated. Likewise, the rectiers R2 and Rs at stations 2 and 3 no longer receive current and the relays in their outputs bccome deenergized thereby bridging the short circuits TC'2 and TCs across the line L. This provides short circuit paths for frequencies f2 and f3 so that these frequencies are no longer transmitted to the station Y and the rectiiiers YR2 and YRa cause the alarms YSz and YSa to be actuated. This indicates at once to the attendant at station Y that the repeater U2 has failed.

If a rectifier such as rectifier R2 should fall while the repeater U2 is still functioning properly, the tuned circuit TC2 would be closed thereby causing the alarm YS2 to indicate a failure of the repeater U2. In this case, however, current will still be transmitted over the line L to the station Y and therefore the alarm Sy would not be operated. The attendant at station Y noting this factwould at once be informed that the trouble was not in the repeater U2, but in the rectifier equipment.

The arrangement, as above described, is intended to provide a continuous circuit for indicating the location of a repeater failure. If it is desired, however, to save the expense of transmitting the frequencies f1, f2, and fa over the line L' at all times, Athe switches associated with the generators of those frequencies at the station X may be opened. In this case the failure of a repeater such as Uz will be indicated at Y by the alarm Sy. The attendant at station Y can then communicate with the attended station X and have the attendant at such station connect the generators XGi, XGa and XGs to the line L'. The fact that frequency f1 is then received and frequencies fz and Ja are not received, will indicate that the repeater Un is the one which has failed.

The arrangement above described provides for transmission in one direction only, and it will be understood that for the path transmitting in the opposite direction the equipment may be duplicated.

Fig. 2, in general, illustrates an arrangement somewhat similar to Fig. 1, except that the test frequencies f1, fz, and f3 are transmitted over the line L itself, instead of over an auxiliary circuit. In this case the rectifiers Rx and Ry with their corresponding alarms Sx and Sy are connected as in Fig. 1. Also, the rectifiers Ri, Rz, and Rs, together with their relays at stations I, 2, and 3, are connected as in Fig. 1. The generators XGi, XG: and XG; at station X are connected to the line L through a filter TFx which passes frequencies fi, f2, and f3 While discriminating against signaling or pilot frequencies transmitted over the line L. A filter SF! is connected to the line L between the connection for the generators and the connection for the rectifier Rx, this filter serving to pass the various signal bands and the pilot frequency, if any, transmitted over. the line L. Filter SFX serves to prevent the frequencies fi, f2, and f3 from being transmitted into the rectifier R! and interfering lwith the proper functioning thereof.

At station I a by-pass for the pilot frequencies fi, f2, and f3 is provided through filters TF1 and TF1 which pass the testing frequencies but discriminate against the signal bands and pilot frequency, if any, normally transmitted over the line L. Filters SFi and SFi are connected in the line L on either side of the repeater Ui within the points at which the by-pass is connected in order to prevent the test frequencies f1, fz, and f: from transmitting through the repeater U1. The relay in the output of the rectifier R1 at station I is arranged to bridge a tuned circuit TCi, tuned to the frequency f1, across the by-pass between the lters TF1 and TF1', in a manner similar to that shown in Fig. l.

At station 2 the by-pass is provided through filters TF2 and TF2' similar to the corresponding filters at station I, and likewise, filters SF2 and SF2 are arranged on either side of the repeater Un to prevent the passage of the testing frequencies. The relay in the output circuit of the rectier Ri also controls a tuned circuit TC: adapted to provide a short circuit for the frequency f2 in connection with the by-pass. Also, at station 3, a by-pass, including filters TF3 and TF3' similar to the corresponding lters at station 2, is provided in the line circuit on either side of the repeater U3, these latter filters being similar to and performing the same function as the corresponding filters in stations I and 2. Also, the relay associated with rectifier R3 controls a circuit tuned to the frequency f: and adapted to be bridged across the by-pass.

At station Y a filter SFy is inserted in the line ahead of the repeater Ay and ahead of the lter a bridged connection is made which leads to nlters YFi, YFz, and YF: and associated rectiflers and alarms similar to those bearing the same reference characters in Fig. 1.

If a repeater such as U2 fails, no signal bands or pilot frequency will be transmitted to rectier Ry, and the alarm Sy will be actuated indicating 5 the failure of some repeater in the line L. Likewise, the rectifiers R2 and Ra will receive no current and their relays will permit their armatures to fall of! causing the tuned circuits TCz and TC: to be bridged across the by-passes at stations 2 10 and 3. This provides short circuits for frequencies fz and .fa which are no longer transmitted to station Y. Consequently, the alarm YSz and YS: will be actuated, indicating that repeater Ua has failed. l5

As in Fig. 1, if merely the rectifier Rz should fail, thereby causing the alarm YSz to be actuated at station Y, the attendant will know that the repeater U2 is still functioning by reason of the fact that the alarms Sy and YS: will not be actuated. Also, as in Fig. 1, the frequencies fi, frand f3 need not be applied to the circuit at all times, but merely switched to the circuit. When an alarm such as Sy is actuated to indicate a failure, as soon as the testing frequencies f1, fz and .f3 are applied to the line after the fact of failure has been noted, the'location of the repeater which has failed will be indicated by the proper alarms YSi, YS: or YS: at station Y.

Fig. 3 shows an arrangement which is similar 30 to that of Fig. 2, in that the line circuit L acts as a transmission path for the testing frequencies f1, fz, and f3. In this case, however, the control over the test frequencies at the unattended repeater station is not effected by means of bridged tuned circuits, but, on the contrary, at each unattended repeater station one of the testing frequencies is passed through the unattended repeater while the other two are by-passed around it. Therefore, when the unattended repeater 0 fails, the particular testing frequency which is permitted to pass through it is no longer transmitted over the line and this causes the operation of an alarm at the attended station. Fig. 3 also differs from Fig. 2 in that an arrangement is 5 provided at each unattended station so that when the repeater fails an auxiliary or substitute repeater is connected in the line automatically.

In Fig. 3 the apparatus at stations X and Y is identical with that of Fig. 2. At station I a. filter TF1' passing frequency f1 is bridged across the output of the repeater U1 and the filter is connected to a rectifler R1 having a relay in its output circuit which is normally energized, so long as the repeater U1 remains operated, to connect the input and output terminals of the repeater in the line circuit L. When the repeater fails, however, the relay is released and its contacts disconnect the repeater U1 and substitute the 30 repeater U1' in the line connections. A filter SF1 is connected in the input line connection to pass the normal signaling bands to the repeater, this filter serving to prevent the passage of the testing frequencies f1, f2, and f3.

The testing frequencies are taken off the line circuit through filters TF1, TF2 and TF3, selective of the frequencies f1, f2, and f3, respectively. The filter TF1 which selects the frequency f1 ,m is connected to the input of the amplifier U1 in parallel with the filter SFi. The filters TF2 and TF3 are selective of frequencies fz and fa, and are connected in the by-pass circuit around the repeater Ui. This by-pass circuit may include, u

ifdesired,zamplifier vB1 for amplifying the frequencies f2 and f3.

The arrangement at station 2 is similar to that at station i, except that at station2 frequencies f1 and f3 are selected and transmitted overthe bypass circuit through the amplifier B2, and frequency f2 is selected by the lter TF2 and applied to the repeater U2. Also, in this case, the filter TF2 bridged across the output of the repeater U2, is selective of frequency f2, and it is the failure of this frequency which causes the operation of the rectifier R2 and its associated relay.

So, also, the arrangement at station 3 is similar to station I, except that frequencies f1 and f2 are selected and transmitted over the by-pass, and

frequency f3 is selected and applied tothe input of the repeater Us. Also, the rectifier R3 is, in this case, responsive to the frequency fs because of the filter TF3 connected in its circuit.

If desired, a single spare amplifier may be arranged Vso that it may replace the normal amplifier foreither direction of transmission. This is indicated by the dotted line connections of U1, U2 and Us inFig. 3.

If in Fig. 3 the repeater U2 should fail at station V2 the frequency ,f2 which is selected out at that station and transmitted through the repeater U2, exclusive of frequencies f1 and f3, will no longer be transmitted. Consequentiy, the frequency f2 will no longer be transmitted through the filter TF2 and the relay associated with said rectifier will release its armatures thereby disconnecting the repeater U2 from the line, and substituting therefor the repeater U2. In making this substitution it Wili be noted that the frequency f2 will not be applied to the repeater U2 as the filter TF2 is connected to the input of the amplier .U2 beyond the switching point. Frequencies f1 and f3 will be transmitted over the line to station Y as before.

At the moment the repeater U2 fails, so that transmission over the line L is interrupted, the alarm Sy at station Y will be actuated momentarily until the repeater U2' is substituted for the repeater U2 so that transmission again takes place. Also, as soon as the repeater U2 fails, so that frequency f2 is no longer transmitted to station Y, the alarm YS2 will be actuated indicating that-the failure has occurred in station 2.

Fig. 4 illustrates an arrangement in Which, as in the other cases, failure of an unattended station is indicated at an attended station by an alarm common to the unattended stations. The three signaling frequencies individual to the three unattended stations may then be applied to the line circuit, one at a time, to determine which repeater has failed. These frequencies are indi- Viduaily selected at the unattended stations and rectified to produce direct currents which are transmitted over the line to indicate whether or not the alternating testing frequency has been transmitted through its repeater. Failure of the repeater is indicated by failure of the direct current indication when the proper test frequency is applied.

In Fig. Li the attended repeaters AX and Ay, their associated rectifiers R1; and Ry, and the alarms SX and Sy are the same as in Figs. 1, 2, and 3.

At station X the generators XG1 XG2 and XGa, generating frequencies f1, f2, and f3, are arranged to be connected to the line, one at a time, by means of a switch. At station l the frequency f1 is selected by means of a filter TF1 connected across the output of the repeater U1, the output of this filter being connected to a rectifier R1 .which vrectiiies the selected frequency and producesa direct current which may be transmitted back tothe line L ahead of the repeater U1 through a filter LP1 which passes direct current but discriminates against alternating current. This causes the direct current to be transmitted vback over the -line L to the bridge circuit in which is connected a filter LPX and a direct current indicator, the filter LPX serving to pass direct current but to reject alternating current.

Similarly, at station 2, the frequency f2 is selected'by filter TF2 and applied to rectifier R2 which is connected through filter LP2 to the line L aheadof the amplifier U2 to permit the rectied'direct current to be transmitted back to station X. So, also, at station 3, the frequency f3 is selected from the output of the repeater Us by filter TF3 Arectified by the rectifier R3 and passedthrough the direct current filter LPs to the line L ahead of Vthe repeater U3.

`in order that the rectified direct current at any .one .ofthe three unattended stations may also be transmitted over the line to the attended stationY, by-passes, including filters LP1, LP2, andLPa arebridged about the repeaters U1, U2

and U3, respectively, these lters serving to pass direct current but rejecting alternating current.

Assuming that the repeater U2 has failed, the signaling 1pand or pilot frequency normally transmitted over the line L is no longer received at station Y and, consequently, the alarm Sy is actuated at said station. The attendant at said station Will then inform the attendant at station X who proceedsto apply frequencies f1, f2, and fa to the line, one at a time, by means of the switch, When frequency f1 is applied to the line the rectifier R1 at station l rectiiies it and causes a direct current to ow back over the line L to operate the Ldirect current indicator at station X, and direct current also flows over the three by-passes to operate the direct current indicator at station Y. When, however, the frequencies f2 or f3 are Japplied to the line this frequency Will not be received by the rectiers R2 or R3 because kof .the Yfailure of the repeater U2, and hence, no rectified current will flow over the line to eitherattended station to operate the direct current indicator. Consequently, the attendant will know that the repeater U2 has failed.

The `received .indication of failure may represent either' the failure of a repeater or the failure of the line section immediately preceding it. It would be possible to distinguish between these tivo Vconditions rby tests made for failure of the line itself in themanner ordinarily employed for telephone cables.

It will be obvious that the general principles hereindisclosed may be embodied in many other organizations widely diiferent from those illustrated, Without departing from the spirit of the `invention vas defined in the claims appended hereto.

What is claimed is:

1. vIn a signaling system, a transmission line having a plurality of unattended repeater stationsbetween attended stations, means to transmit a plurality of alternating currents of difierent-frequencies from an attended station to said unattended repeater stations, means whereby the failure of a repeater at an unattended repeater station produces aneffect upon at least one of -said alternating currents distinguishable from the effect produced by the failure of the repeaters at other unattended stations, and means at an attended station controlled by said alternating currents to indicate the repeater station at which the failure has taken place.

2. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit a plurality of alternating currents of different frequencies from an attended station to said unattended repeater stations, means whereby the failure of a repeater at an unattended repeater station produces an effect upon at least one of said alternating currents distinguishable from the effect produced by the failure of the repeaters at other unattended stations, and means at an attended station controlled by said alternating currents to indicate that an unattended repeater has failed and to indicate the repeater station at which the failure has occurred.

3. In a' signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit a plurality of alternating currents of dierent frequencies from an attended station to said unattended repeater stations, means whereby the failure of a repeater at an unattended repeater station produces an effect upon at least one of said alternating currents distinguishable from the effect produced by the failure of the repeaters at other unattended stations, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at an attended station controlled by said alternating currents to indicate the repeater station at which the failure has taken place.

4. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit a plurality of alternating currents of different frequencies from an attended station to said unattended repeater stations, means whereby the failure of a repeater at an unattended repeater station produces an effect upon at least one of said alternating currents distinguishable from the effect produced by the failure of the repeaters at other unattended stations, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at an attended station controlled by said alternating currents to indicate the repeater station at which the,

failure has taken place.

5. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit from an attended station alternating currents of different frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means whereby the failure of a repeater at an unattended repeater station produces an effect upon the alternating current of frequency corresponding to that station, and means at an attended station responsive to such effect'to indicate the repeater station at which the failure takes place.

6. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit from an attended station alternating currents of different frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means whereby the failure of a repeater at an unattended repeater station produces an effect upon the alternating current of frequency corresponding to that station, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at an attended station responsive to such effect to indicate the repeater station at which the failure takes place.

'7. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit a plurality of altem-ating currents of different frequencies from an attended station to said unattended repeater stations, means whereby the failure of a repeater at an unattended repeater station produces an effect upon at least one of said alternating currents distinguishable from the effect produced by the failure of the repeaters at other unattended stations, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at each unattended repeater station operating upon the failure of a repeater thereat to produce a change in the transmission of the alternating current whose frequency corresponds to that station without changing the transmission of the alternating currents corresponding to other stations, and means at an attended station responsive to such change to indicate the repeater station at which failure takes place.

8. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to transmit a plurality of alternating currents of different frequencies from an attended station to said unattended repeater stations, means whereby the failure of a repeater at an unattended repeater station produces an effect upon at least one of said alternating currents distinguishable from the effect produced by the failure of the repeaters at other unattended stations, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, means at each unattended repeater station operating upon the failure of a repeater thereat to produce a change in the transmission of the alternating current whose frequency corresponds to that station without changing the transmission of the alternating currents corresponding to other stations, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at an attended station responsive to such change to indicate the repeater station at which failure takes place.

9. In a signaling system, a transmission line n having a plurality of unattended repeater stations between attended stations, means to superpose on said transmission line test channels for the transmission of alternating currents of different frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means at each unattended repeater station to separate said test channels from the line circuit, selective means responsive to failure of a repeater at an unattended station to change the transmission of the frequency of the test channel corresponding to that station without affecting the frequencies corresponding to other repeater stations. and means at an attended station responsive to such change to indicate the repeater station at which failure takes place.

l0. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to superpose on said transmission line test channels for the transmission of alternating currents of different frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means at each unattended repeater station to separate Said test channels from the line circuit, selective means responsive to failure of a repeater at an unattended station to change the transmission ofthe frequency of the test channel corresponding to that station without affecting the frequencies corresponding to other repeater stations, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at an attended station responsive to such change to indicate the repeater station at which failure takes place.

11. In a signaling system, a transmission line having aplurality of unattended repeater stations between attended stations, means to superpose on said transmission line test channels for the transmission of alternating currents of difyferent frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means at each unattended repeater station to by-pass `around the repeater thereat frequencies of test channels-corresponding to other unattended repeater stations so that the failureof any unattended repeater will causefailure of the alternating current whose frequency corresponds to that station, and means at an'attended station responsive to the failure of the frequency correspondingto an unattended repeater station'to indicate that a repeater has failedat such'unattended station.

12; In a signaling system, a'transmission line having a plurality of unattended'repeater stations between attended stations, means to superpose on said transmission linetest channels for thek transmission of alternating-currents of different frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means at yeachy unattended repeater station to by-pass around the repeaters thereat frequencies of test channels corresponding to other unattended repeater stations so that the failure of any unattended repeater will cause failure of the alternating current whose frequency corresponds to that station, means associated withthe transmission' line at an attended station to indicate that a failure has occurred ahead of the attended station, and means at an attendedstation responsive to the failure-of the frequency corresponding to an'unattended repeater station to indicate that aA repeater has failed at such unattended station.

13. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means to superpose von said transmission line test channels for the transmission of alternating currents of different frequencies to said unattended repeater stations, there being one frequency corresponding to each unattended repeater station, means at each unattended repeater station to by-pass around the repeaters thereat frequencies of test channels corresponding to other unattended repeater stations so that the failure of any unattended repeater will cause failure of the alternating current whose frequency corresponds to that station, means associated with the transmis- 5 sion line at an attended station to indicate that a failure has occurred ahead of the attended station, means at an attended station responsive to the failure of the frequency corresponding to an unattended repeater station to indicate that a l` repeater has failed at such unattended station, and means at each unattended repeater station operatingA in response to the failure of the frequency corresponding thereto to substitute an alternative repeater at such station. l 14. In a signaling system, a transmission line having a plurality of unattended repeater stations between attended stations, means at an attended station to supply alternating test currents of frequencies corresponding to each unattended repeater station, means to apply an alternating current of frequency corresponding to any unattended repeater station to said transmission line, means at each unattended repeater station to change an alternating test current re- 28 ceived thereat into a direct current, means to transmit such direct current to an attended station, and indicating means at such attended station to indicate the transmission of direct current thereto, whereby failure of direct current 30- wnen an alternating test current corresponding to a particular unattended repeater has been applied to said transmission line will indicate failure of a repeater at such unattended station.

15. In a signaling system, a transmission line $5 having a plurality of unattended repeater stations between attended stations, means at an attended station to supply alternating test currents of frequencies corresponding to each unattended repeater station, means to apply an alternating currentof frequency corresponding to any unattended repeater station to said transmission line, means at each unattended repeater station to-change an alternating test current received thereat into a direct current, means to transmit such direct current to an attended station, means associated with the transmission line at an attended station to indicate that a failure has occurred ahead of the attended station, and indicating means at such attended station to indil cate the transmission of direct current thereto, whereby failure of direct current when an alternating test current corresponding to a particular unattended repeater has been applied to said transmission line will indicate failure of a 5 5 repeater at such unattended station.

16. In a signaling system, a transmission line extending between two attended stations through a plurality of unattended repeater stations, means at each unattended station for automatically switching in a spare amplifier to replace an amplier which fails, means common to several unattended stations to indicate at an attended station when an amplifier has failed, and means further to indicate the unattended station at which 05 such failure has occurred.

ESTILL I. GREEN. FRANK A. LEIBE. 

